The subject matter disclosed herein relates to an MRI apparatus (Magnetic Resonance Imaging) apparatus and an RF (radio frequency) transmission gain setting method, and more particularly to an MRI apparatus which obtains tomograms of the object by utilizing magnetic resonance and an RF transmission gain setting method for MRI apparatuses.
In an MRI apparatus, a prescan is done before taking an image, and calibration is performed to optimize the central frequency and the gain of RF transmission by using the imaged data. By the calibration, the center frequency of RF transmission is so adjusted as to be identical with the Larmor frequency of the spin of the object, and the gain of RF transmission is so adjusted as to set the flip angle of spin excitation exactly to a prescribed angle (see U.S. Pat. No. 4,806,866 and/or U.S. Pat. No. 6,025,718, for instance).
In an MRI apparatus that generates its main magnetic field with a permanent magnet requires frequent prescanning for calibration because the influence of heating of the gradient magnet field coil causes the magnetic field intensity to drift dependent on the temperature characteristics of the permanent magnet and along with that the Larmor frequency varies. This extends the time required for imaging, with a correspondingly increased load on the patient and inconvenience to the user.